Calendering is an economic and highly efficient means to produce film and sheet from plastics such as plasticized and rigid poly (vinyl chloride) (PVC) compositions. The films and sheets usually have a thickness ranging from about 2 mils (0.05 mm) to about 45 mils (1.14 mm). They are readily thermoformed into various shapes and are used for a wide variety of packaging applications. Calendered PVC film or sheet can be used in a wide range of applications including pool liners, graphic arts, transaction cards, security cards, veneers, wall coverings, book bindings, folders, floor tiles and products which are printed or decorated or laminated in a secondary operation.
Japan Application No. Heisei 7-197213 (1995) to E. Nishimura et al. and European Patent Application 0 744 439 A1 (1996) to Y. Azuma et al. disclose the state of the art with regard to polypropylene resin compositions used in calendering processes.
In a typical calendering process line, the plastic resin is blended with specific ingredients such as stabilizers to prevent thermal degradation; modifiers for clarity, heat stability or opacity characteristics; pigments; lubricants and processing aids; anti-static agents; UV inhibitors; and flame retardants. The mixed ingredients are plasticized in a kneader or extruder. Through heat, shear and pressure, the dry powders are fused to form a homogeneous, molten material. The extruder feeds the molten material in a continuous process to the top of the calendering section of the calendering line in between first and second heated calender rolls. Typically, four rolls are used to form three nips or gaps. The rolls are configured in an "L" shape or an inverted "L" shape. The rolls vary in size to accommodate different film widths. The rolls have separate temperature and speed controls. The material proceeds through the nip between the first two rolls, referred to as the feed nip. The rolls rotate in opposite directions to help spread the material across the width of the rolls. The material winds between the first and second, second and third, third and fourth rolls, etc. The gap between rolls decreases in thickness between each of the rolls so that the material is thinned between the sets of rolls as it proceeds. After passing through the calender section, the material moves through another series of rolls where it is stretched and gradually cooled forming a film or sheet. The cooled material is then wound into master rolls. General descriptions of calendering processes are disclosed in Jim Butschli, Packaging World, p. 26-28, June 1997 and W. V. Titow, PVC Technology, 4.sup.th Edition, pp 803-848 (1984), Elsevier Publishing Co., both incorporated herein by reference.
Although PVC compositions are by far the largest segment of the calendered film and sheet business, small amounts of other thermoplastic polymers such as thermoplastic rubbers, certain polyurethanes, talc-filled polypropylene, acrylonitrile/butadiene/styrene terpolymers (ABS resins) and chlorinated polyethylene are sometimes processed by calendering methods. Attempts to calender polyester polymers such as poly(ethylene terephthalate) (PET) or poly(1,4-butylene terephthalate) (PBT) have not been successful. For example, PET polymers with inherent viscosity values of about 0.6 dL/g have insufficient melt strength to perform properly on the calendering rolls. Also when the polyester is fed to the rolls at typical processing temperatures of 160.degree. C. to 180.degree. C., the PET polymer crystallizes causing a non-homogeneous mass which is unsuitable for further processing. The non-homogeneous mass causes undesirable high forces on the calender bearings. The tendency of polyester polymers to hydrolyze during processing in the molten or semi-molten state on rolls open to ambient conditions is also a concern. Typical PET polymers without the inclusion of process lubricants or internal release additives, also have a tendency to stick to the calendering rolls at typical processing temperatures.
Conventional processing of polyesters into film or sheet involves extruding a polyester melt through a manifold of a flat die. Manual or automatic die lip adjustment is used to control thickness across a web of material. Water-cooled chill rolls are used to quench the molten web and impart a smooth surface finish. A typical die extrusion process is shown in FIGS. 2A and 2B. Extrusion processes while producing film and sheet of excellent quality do not have the throughput and economic advantages that are provided by calendering processes.
Thus, there exists a need in the art for an efficient and economic process to manufacture polyester films and sheets as an alternative to extrusion processes. Accordingly, it is to the provision of such that the present invention is primarily directed.